From the prior art, methods are known for converting silicon halides, which optionally can have additional hydrocarbon groups, into the corresponding silanes by reaction with magnesium hydride. However, these known methods are less suitable for use on an industrial scale, since they proceed either at high temperatures or only after chemical activation of the magnesium hydride. However, in the activated form, magnesium hydride generally is pyrophoric, so that special safety precautions must be observed during its use.
For example, it is known from the German Offenlegungschrift 32 47 362 that silicon hydrides, particularly SiH.sub.4, is prepared by reacting halogen silanes in a solvent with magnesium hydride, which was obtained by reacting magnesium in the presence of a catalyst, consisting of a halide of a metal of the subsidiary group IV to VIII of the periodic table and an organomagnesium compound or a magnesium hydride, as well as optionally in the presence of a polycyclic aromatic compound or a tertiary amine as well as optionally in the presence of a magnesium halide MgX.sub.2 with X= Cl, Br or I, with hydrogen. The synthesis of this magnesium hydride is expensive and, because it ignites spontaneously, it can be handled only if special precautionary measures are observed. Moreover, the method proceeds only with yields of, at most, 80% of the theoretical yield.
In the German Offenlegungsschrift 34 09 172, a method is described for the synthesis of SiH.sub.4. For this method, SiF.sub.4 is reacted with magnesium hydride in a melt of alkali halides or alkaline earth halides under a hydrogen partial pressure, which is greater than the dissociation pressure of the magnesium hydride at the temperature of the melt. The large amount of energy required to melt the eutectic salt system, which is employed here and has a melting point between 318.degree. and 450.degree. C., negates the advantage of a solvent-free system. Furthermore, the method cannot be used with organosilicon halides, since at the very least a partial destruction of the organic substituents must be expected here.
The German patent 36 37 273 relates to a method for synthesizing organopolysiloxanes containing SiH groups from the corresponding organopolysiloxanes containing silicon halide groups by the reaction of metal hydrides in a liquid reaction medium. The method has the following distinguishing features:
a) use of a metal hydride from group consisting of LiH, NaH, KH, CaH.sub.2 and MgH.sub.2 ; PA1 b) use of conventional ethers, particularly tetrahydrofuran, as reaction medium; PA1 c) continuous removal of metal halide formed being deposited on PA1 c) continuous removal of metal halide formed being deposited on the surface of the metal hydride particles during the reaction with formation of a fresh surface by the action of mechanical energy or ultrasound. PA1 a) the use of non-pyrophoric storage magnesium hydride as magnesium hydride, PA1 b) the use of conventional ethers as reaction medium, and PA1 c) the continuous removal of the magnesium halide formed being deposited on the surface of the magnesium hydride particles during the reaction by the action of mechanical energy or ultrasound so as to form a fresh surface. PA1 R.sup.1 in the molecule is the same or different and represents an optionally halogenated hydrocarbon group with up to 24 carbon atoms, PA1 X is a halogen group, and PA1 p is a number from 0 to 3. PA1 R.sup.1 and X have the meanings already given, PA1 R.sup.2 is a divalent hydrocarbon group with 2 to 24 carbon atoms, and PA1 is a number from 0 to 2.
A transfer of this method to monomeric chlorosilanes or organochlorosilanes was not obvious to those skilled in the art, since it was known that organochlorosilanes, because of their higher oxygenophilicity, can react with ethers by splitting the ether bond.
Attempts have therefore been made recently to find alternative methods, in order to synthesize the desired silanes from the corresponding halogen silanes. One such alternative method was described in the German Offenlegungsschrift 40 32 168, where an organosilicon halide is converted with aminalane, such as triethylaminalane, into the corresponding organosilicon hydride. This aminalane is synthesized by reacting a solution of triethylamine and AlCl.sub.3 in toluene with NaAlH.sub.4. This method also has the disadvantage that it can be carried out only on a laboratory scale. On an industrial scale, the use of NaAlH.sub.4 is much too expensive and would require major safety precautions.